This invention relates to polyimide film materials and processes of producing these materials by solvent casting.
Aromatic polyimides have been used in high temperature applications, since these materials retain high strength under continuous use at temperatures up to about 300.degree. C. and even higher for brief periods. Continuous films, foils, sheets, cloth and/or laminates are particularly desirable for use in high temperature electrical insulation due to the thermal stability and resistance of these resins. Fully polymerized imide resins require no post cure to develop the high temperature properties. Solution spinning or casting techniques can be employed for making various stock shapes in continuous manufacturing processes, using commercially available solutions of polyimides in polar solvents, such as dimethylformamide.
While imide resin substrates may be employed for flame-resistant circuit boards, radomes, etc., a large use is found in thin film for wrapping electrical motors or the like, where these materials can withstand long-term high temperatures without loss of mechanical or electrical properties. Thermoset polyimides are particularly valuable as films, with fused-ring aromatic moieties contributing high thermal stability. High tensile strength over a wide temperature range, dimensional stability, wear resistance, high dielectric strength, chemical resistance, and radiation resistance are properties which are desirable for the many uses of polyimides. Polyimide film, such as "Kapton", has found use in compact electrical motors, where high dielectric strength, and toughness are important, as well as in insulation for aircraft and missile cable, etc. In wrapping insulation film, flexibility and elongation of the film are important to permit the polyimide to conform to the substrate shape. Films made by solution casting can be oriented after removing at least a portion of the solvent, as by evaporation from the film. Molecular orientation may be effected by stretching the film in machine direction orientation (MDO) and/or transverse direction orientation (TDO) at orientation temperature. Typical prior films made by this process may have 10% to 25% elongation before breaking under tensile stress.